The present invention generally relates to methods of correcting output waveforms of heads and magnetic disk units, and more particularly to a method of correcting an output waveform of a magneto-resistance (MR) head which is made up of a MR element that uses the magneto-resistive effect and outputs a waveform which is vertically asymmetrical about a horizontal axis, and to a magnetic disk unit which employs such a method of correcting the output waveform of the MR head.
In this specification, "a waveform which is vertically asymmetrical about the horizontal axis" is intended to mean that a waveform has a first portion on a positive polarity side of the vertical axis and a second portion on a negative polarity side of the vertical axis, and that the waveforms of the first and second portions are asymmetrical about the horizontal axis when the time base of at least one of the first and second portions is shifted to see if the first portion is a mirror image of the second portion with respect to the horizontal axis.
Recently, in magnetic disk units, the recording density of a magnetic disk has improved notably. Hence, a MR head made up of a MR element having a resistance which changes depending on the magnetic flux density has been proposed for use as a read head in order to cope with the improved recording density of the magnetic disk.
When reproducing recorded information from the magnetic disk using the MR head, a constant bias current is applied to the MR head. And when the magnetic flux density changes depending on the information recorded on the magnetic disk, the resistance of the MR element forming the MR head changes in accordance with the change in the magnetic flux density. As a result, the information read from the magnetic disk is output as a voltage via terminals of the MR head.
The output of the MR head that is obtained when reproducing the information recorded on the magnetic disk has a waveform that is vertically asymmetrical about the horizontal axis due to the characteristic of the MR head. In other words, the signal amplitude on a positive polarity side differs from that on a negative polarity side of the output waveform of the MR head. The vertical asymmetry of this vertically asymmetrical output waveform of the MR head differs depending on each individual MR head and is not constant even for the same make of MR heads.
The output waveform of the MR head is subjected to an analog-to-digital conversion process, a most likelihood decoding process and the like before the information recorded on the magnetic disk is finally reproduced. Hence, if the vertical asymmetry of the output waveform of the MR head is large, a sample error of the analog-to-digital conversion process occurs, that is, a difference between a sample value and a target value becomes large. In this case, an error is easily generated at a latter stage where the most likelihood decoding process is carried out, thereby causing an increase in the error rate of the magnetic disk unit.
The vertical asymmetry of the output waveform of the MR head tends to decrease when the bias current or sense current applied to the MR head increases. For this reason, the vertical asymmetry of the output waveform of the MR head is conventionally suppressed to a small value by increasing the bias current, that is, adjusting the bias current so that the error rate of the magnetic disk unit falls within a prescribed value.
However, when the bias current applied to the MR head increases, the bias current in most cases exceeds a prescribed value of the MR head and shortens the serviceable life of the MR head. In addition, in some MR heads, the vertical asymmetry of the output waveform of the MR head does not show a considerable improvement even if a large bias current is applied to the MR head.
On the other hand, when the bias current applied to the MR head does not exceed the prescribed value of the MR head, the vertical asymmetry of the output waveform of the MR head does not show a considerable improvement in most cases, and there was a problem in that it is difficult to improve the error rate of the magnetic disk unit in such cases.